1. Field of the Invention
The present invention relates to a magnetooptical recording medium for recording and reproducing data by utilizing a magnetooptical effect by means of irradiation of a laser beam, and to a method and apparatus for recording and reproducing data to and from the same.
2. Description of the Related Art
Optical disks or the like for writing and reading data by means of a laser beam are known to be superior in recording density and storage capacity.
As an optical recording medium such as the rewritable optical disk and the like, magnetooptical ones utilizing the magnetic Kerr effect have been used in general. FIG. 1 is a schematic cross-sectional view of a conventional magnetooptical recording medium of the type described in, for example, SPIE Proceeding, Vol. 1316, pp. 81 to 90, 1990.
The magnetooptical recording medium shown in FIG. 1 comprises a transparent substrate 101, and a transparent interference layer 102, a first magnetooptical layer 103 made of a ferrimagnetic amorphous alloy of GdFeCo with a high Curie temperature, a second magnetooptical layer 104 made of a perpendicularly magnetizable ferrimagnetic amorphous alloy of TbFe with a low Curie temperature, and a dielectric protective layer 106 which are laminated in this order on the substrate 101.
The first magnetooptical layer 103 has a comparatively small coercive force at a temperature at which readout is executed, and the second magnetooptical layer 104 has a comparatively large coercive force at the temperature at which readout is executed. Besides, the first and second magnetooptical layers 103 and 104 are exchange-coupled at the temperature at which readout is executed.
A laser beam for writing or reading use is incident through the transparent substrate 101 to the magnetooptical recording medium so as to be focused in a diameter of about 1.4 .mu.m in the vicinity of the first and second magnetooptical layers 103 and 104 by means of a focusing servo mechanism (not shown). As to the laser source, a semiconductor laser generating a laser beam having a wavelength (.lambda.) of about 8300 angstroms is employed, and an objective lens having a numerical aperture (NA) of about 0.55 is also employed.
In order to write data thereinto, the second magnetooptical layer 104 is heated up to the vicinity of its Curie temperature depending on the data to be written and a recording bias magnetic field is applied to the region including this heated area, so that magnetization of the area heated to the vicinity of the Curie temperature is oriented to the direction opposite to the direction of the magnetization of the other area. When the temperature is lowered, the reverse magnetic domains written into the second layer are transferred to the first magnetooptical layer 103.
In order to read the data therefrom, a focused laser beam polarized substantially linearly is irradiated to the first magnetooptical layer 103 while being relatively moved thereto, and the beam reflected therefrom is detected optically by means of an analyzer. The magnetooptical film has an effect to rotate the polarization plane of the reflected beam through the magnetic Kerr effect. The rotational angle .theta.k of the polarization plane of the reflected beam differs depending on the direction of perpendicular magnetization of the magnetooptical film, so that by passing the reflected beam through the analyzer before being entered into an optical detector, the data corresponding to the direction of magnetization can be read out as a change of light intensity.
Since such a magnetooptical film as described above is extremely susceptible to oxidization, it is sandwiched between the transparent interference layer 102 and dielectric protective layer 106 for effectively avoiding oxidization.
With the above-mentioned conventional magnetooptical recording medium and recording and playback method, however, a limitation is imposed thereon in recording density. This is because the resolution of a reproduced signal obtained by reading optically the pattern of the reverse magnetic domains, which is the recorded data, depends on the wavelength (.lambda.) of a laser beam of the laser source of the optical system for reproduction and on the numerical aperture (NA) of an objective lens, as is evidenced by the period of the reverse magnetic domains being expressed in about .lambda./(2.times.NA) as its detection limit. Hence, in order to realize high density recording and reproducing in the magnetooptical recording medium, it is necessary to shorten the wavelength (.lambda.) of the source beam and increase the numerical aperture (NA) of the objective lens.
However, such a laser beam source that can reliably irradiate a laser beam having a short wavelength is difficult to develop because of short life thereof. Besides, it is difficult to increase the numerical aperture of the objective lens because of such a problem that the magnetooptical recording medium itself may be deflected, or other problems may arise therein.
In consideration of those problems, many attempts have been made on the realization of high recording density through improving the magnetooptical recording medium itself and the method for reading data therefrom. For example, a magnetooptical recording medium and playback method are disclosed in Japanese Patent Laid-Open Publication NO. 3-242845, in which a written pattern of reverse magnetic domains is deformed and read out by utilizing the temperature rise of a magnetooptical layer through irradiating a laser beam for reading use, thus providing enough reproduction output from the reverse magnetic domains even below the limit in optical detection.
In the method as mentioned above, the magnetooptical recording medium has, as cross-sectionally shown in FIG. 2, a transparent substrate 111, and a transparent interference layer 112, a first magnetooptical layer 113, a second magnetooptical layer 114, a third magnetooptical layer 115 and a dielectric protective layer 116 laminated in this order on the substrate 111.
Each of the first magnetooptical layer 113, second magnetooptical layer 114 and third magnetooptical layer 115 is a perpendicularly magnetizable film made of an alloy of iron series transition metal and rare earth transition metal. The first, second and third magnetooptical layers 113, 114 and 115 are exchange-coupled at room temperature, and it is preferable that the first magnetooptical layer 113 has a film thickness of 250 angstroms or above, and the second magnetooptical layer 114 has a film thickness of 50 angstroms or above.
The above-mentioned publication discloses that the transparent substrate 111 is a glass 2P substrate, the transparent interference layer 112 is a Si.sub.3 N.sub.4 film with a thickness of 800 angstroms, the first magnetooptical layer 113 is a GdFeCo film with a thickness of 300 angstroms, the second magnetooptical layer 114 is a TbFe film with a thickness of 150 angstroms, the third magnetooptical layer 115 is a TbFeCo film with a thickness of 550 angstroms, and the dielectric protective layer 116 is a Si.sub.3 N.sub.4 film with a thickness of 800 angstroms.
FIG. 3 shows a well-known writing method generally applied to the magnetooptical recording medium of the type described in the publication as mentioned above, in which the magnetooptical recording medium is cross-sectionally shown at a state where the pattern of reverse magnetic domains is formed by the well-known writing method. In FIG. 3, the arrows in the films indicate the directions of magnetization. FIG. 4 is a top plan view of the magnetooptical recording medium as observed from the transparent substrate side. When the radius of the light beam for reading use is larger than the pitch of the reverse magnetic domains MK since there exist plural reverse magnetic domains within a beam LB of the laser beam for reading use, the reverse magnetic domains MK cannot be individually read out by the playback method applied to the magnetooptical recording medium in the prior art. (In FIG. 4, each of the regions where magnetization is directed upward is shown by the right ascendant oblique lines, and its shape is schematically shown in circle.)
FIG. 5 shows a reading method applied to reverse magnetic domains shown in FIG. 3. In FIG. 5, by increasing the temperature in a region HT shown by the right descendent oblique lines up to the Curie temperature T.sub.C2 or above of the second magnetooptical layer 114, the magnetization of the second magnetooptical layer 114 is substantially diminished, so that the exchange coupling between the first and third magnetooptical layers 113 and 115 is disengaged. As shown above, two reverse magnetic domains are discriminated by using the fact that the temperature of the magnetooptical film of the front area HT (first region, shown by right descendent oblique lines) is increased when observed in the direction that the magnetooptical recording medium is relatively moved.
FIG. 6 is a schematic cross-sectional view of the magnetooptical recording medium under the state shown in FIG. 5. If an external magnetic field H.sub.PB larger than the coercive force of the first magnetooptical layer 113 is applied under this state, the direction of magnetization of the first magnetooptical layer 113 is aligned with the direction of the external magnetic field H.sub.PB in the region HT shown by the right descendent oblique lines.
On the other hand, in the second region other than the right descendent oblique line region HT, that is, in the region where the temperature is lower than the Curie temperature T.sub.C2, the magnetic coupling between the first and third magnetooptical layers 113 and 115 is maintained, so that the pattern of the reverse magnetic domains written in the third magnetooptical layer 115 is transferred to the first magnetooptical layer 113 and held therein.
FIG. 7 is a top plan view of the magnetooptical recording medium as observed from the transparent substrate side. As shown in FIG. 7, the reverse magnetic domain MK2 on the front side within the beam diameter of a laser beam for reading use is "masked", so that the reverse magnetic domain MK2 written In this area is apparently diminished, hence it appears that there exists only one reverse magnetic domain MK1 within the beam diameter thereof. As a result, the spatial frequency of the written reverse magnetic domains as observed through the laser beam for reading use appears lower than the actual frequency although the written reverse magnetic domains remain unchanged, resulting in an improvement in resolution during reproduction.
With the magnetooptical recording medium and recording and playback method in the prior art as mentioned above, however, such disadvantages are pointed out that the recording density is not always satisfactorily high and that bit error rate and weather resistance are inferior.